Image display system

ABSTRACT

A wearable image display system includes a headpiece, a first and a second light engine, and a first and a second optical component. The first and second light engines generate a first and a second set of beams respectively, each beam substantially collimated so that the first and second set form a first and a second virtual image respectively. Each optical component is located to project an image onto a first and a second eye of a wearer respectively. The first and second sets of beams are directed to incoupling structures of the first and second optical components respectively. Exit structures of the first and second optical components guide the first and second sets of beams onto the first and second eyes respectively. The optical components are located between the light engines and the eyes. Both of the light engines are mounted to a central portion of the headpiece.

RELATED APPLICATION

This application is a continuation of and claims priority to U.S. patentapplication Ser. No. 14/617,769, filed Feb. 9, 2015, entitled “WearableImage Display System”, the entire disclosure of which is herebyincorporated by reference herein in its entirety.

BACKGROUND

Display systems can used to make a desired image visible to a user(viewer). Wearable display systems can be embodied in a wearable headsetwhich is arranged to display an image within a short distance from ahuman eye. Such wearable headsets are sometimes referred to as headmounted displays, and are provided with a frame which has a centralportion fitting over a user's (wearer's) nose bridge and left and rightsupport extensions which fit over a user's ears. Optical components arearranged in the frame so as to display an image within a few centimetresof the user's eyes. The image can be a computer generated image on adisplay, such as a micro display. The optical components are arranged totransport light of the desired image which is generated on the displayto the user's eye to make the image visible to the user. The display onwhich the image is generated can form part of a light engine, such thatthe image itself generates collimated lights beams which can be guidedby the optical component to provide an image visible to the user.

Different kinds of optical components have been used to convey the imagefrom the display to the human eye. These can include lenses, mirrors,optical waveguides, holograms and diffraction gratings, for example. Insome display systems, the optical components are fabricated using opticsthat allows the user to see the image but not to see through this opticsat the “real world”. Other types of display systems provide a viewthrough its optics so that the generated image which is displayed to theuser is overlaid onto a real world view. This is sometimes referred toas augmented reality.

Waveguide-based display systems typically transport light from a lightengine to the eye via a TIR (Total Internal Reflection) mechanism in awaveguide (light guide). Such systems can incorporate diffractiongratings, which cause effective beam expansion so as to output expandedversions of the beams provided by the light engine. This means the imageis visible over a wider area when looking at the waveguide's output thanwhen looking at the light engine directly: provided the eye is within anarea such that it can receive some light from substantially all (i.e.all or most) of the expanded beams, the whole image will be visible tothe user. Such an area is referred to as an eye box.

In one type of head mounted display, the frames support two lightengines, which each generate an image for a respective eye, withrespective guiding mechanisms which each guide the image to project itat a proper location with respect to the associated eye so that thewearer's eyes operate in unison to receive a single non-distorted image.

SUMMARY

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the detaileddescription. This summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used to limit the scope of the claimed subject matter. Nor is theclaimed subject matter limited to implementations that solve any or allof the disadvantages noted in the background section.

A wearable image display system comprises a headpiece, a first and asecond light engine, and a first and a second optical component. Thefirst and second light engines are configured to generate a first and asecond set of beams respectively. Each beam is substantially collimatedso that the first and second set form a first and a second virtual imagerespectively. The light engines are mounted on the headpiece. Eachoptical component is located to project an image onto a first and asecond eye of a wearer respectively and comprises an incouplingstructure and an exit structure. The first and second sets of beams aredirected to the incoupling structures of the first and second opticalcomponents respectively. The exit structures of the first and secondoptical components are arranged to guide the first and second sets ofbeams onto the first and second eyes respectively. The opticalcomponents are located between the light engines and the eyes. Both ofthe light engines are mounted to a central portion of the headpiece.

BRIEF DESCRIPTION OF FIGURES

FIG. 1 shows a wearable display system;

FIG. 2A shows a plan view of part of the display system;

FIG. 2B shows a plan view of the display system;

FIGS. 3A and 3B shows perspective and frontal view of an opticalcomponent;

FIG. 4A shows a schematic plan view of an optical component having asurface relief grating formed on its surface;

FIG. 4B shows a schematic illustration of the optical component of FIG.4A, shown interacting with incident light and viewed from the side;

FIG. 5A shows a schematic illustration of a straight binary surfacerelief grating, shown interacting with incident light and viewed fromthe side;

FIG. 5B shows a schematic illustration of a slanted binary surfacerelief grating, shown interacting with incident light and viewed fromthe side;

FIG. 5C shows a schematic illustration of an overhanging triangularsurface relief grating, shown interacting with incident light and viewedfrom the side;

FIG. 6 shows a close up view of part of an incoupling zone of an opticalcomponent;

FIG. 7A shows a perspective view of a part of a display system;

FIG. 7B shows a plan view of individual pixels of a display;

FIGS. 7C and 7D show plan and frontal views of a beam interacting withan optical component;

FIG. 7E shows a frontal view of an optical component performing beamexpansion;

FIG. 7F shows a plan view of an optical component performing beamexpansion;

FIG. 7G is a plan view of a curved optical component;

FIGS. 8A and 8B are plan and frontal views of a part of an opticalcomponent;

FIG. 9A shows a perspective view of beam reflection within a fold zoneof a waveguide;

FIG. 9B illustrates a beam expansion mechanism;

FIG. 10 shows a side view of a display system;

FIG. 11 shows how ghost images may be created in certain displaysystems;

FIG. 12 illustrates a mechanism by which ghost images can be eliminated.

DETAILED DESCRIPTION

Typically, a waveguide based display system comprises an image source,e.g. a projector, waveguide(s) and various optical elements (e.g.diffraction gratings or holograms) imprinted on the waveguide surfaces.The optical elements are used, for example, to couple light emitted bythe image source into and out of the waveguide, and/or for manipulationof its spatial distribution within the waveguide.

FIG. 1 is a perspective view of a head mounted display. The head mounteddisplay comprises a headpiece, which comprises a frame (2) having acentral portion (4) intended to fit over the nose bridge of a wearer,and a left and right supporting extension (6, 8) which are intended tofit over a user's ears. Although the supporting extensions are shown tobe substantially straight, they could terminate with curved parts tomore comfortably fit over the ears in the manner of conventionalspectacles.

The frame 2 supports left and right optical components, labelled 10L and10R, which are waveguides e.g. formed of glass or polymer. For ease ofreference herein an optical component 10 (which is a waveguide) will beconsidered to be either a left or right component, because thecomponents are essentially identical apart from being mirror images ofeach other. Therefore, all description pertaining to the left-handcomponent also pertains to the right-hand component. The opticalcomponents will be described in more detail later with reference to FIG.3. The central portion (4) houses two light engines which are not shownin FIG. 1 but one of which is shown in FIG. 2A.

FIG. 2A shows a plan view of a section of the top part of the frame ofFIG. 1. Thus, FIG. 2A shows the light engine 13 which comprises a microdisplay 15 and imaging optics 17 including a collimating lens 20. Thelight engine also includes a processor which is capable of generating animage for the micro display. The micro display can be any type of imagesource, such as liquid crystal on silicon (LCOS) displays transmissiveliquid crystal displays (LCD), matrix arrays of LED's (whether organicor inorganic) or any other suitable display. The display is driven bycircuitry which is not visible in FIG. 2A which activates individualpixels of the display to generate an image. The substantially collimatedlight, from each pixel, falls on an exit pupil 22 of the light engine13. At exit pupil 22, collimated light beams are coupled into eachoptical component, 10L, 10R into a respective in-coupling zone 12L, 12Rprovided on each component. These in-coupling zones are clearly shown inFIG. 1, but are not readily visible in FIG. 2A. In-coupled light is thenguided, through a mechanism that involves diffraction and TIR, laterallyof the optical component in a respective intermediate (fold) zone 14L,14R, and also downward into a respective exit zone 16L, 16R where itexits the component 10 towards the users' eye. The zones 14L, 14R, 16Land 16R are shown in FIG. 1. These mechanisms are described in detailbelow. FIG. 2A shows a user's eye (right or left) receiving thediffracted light from an exit zone (16L or 16R). The output beam OB to auser's eye is parallel with the incident beam IB. See, for example, thebeam marked IB in FIG. 2A and two of the parallel output beams marked OBin FIG. 2A. The optical component 10 is located between the light engine13 and the eye i.e. the display system configuration is of so-calledtransmissive type.

The optical component 10 is substantially transparent such that a usercan not only view the image from the light engine 13, but also can viewa real world view through the optical component 10.

The optical component 10 has a refractive index n which is such thattotal internal reflection takes place guiding the beam from thein-coupling zone 12 along the intermediate expansion zone 14, and downtowards the exit zone 16.

FIG. 2B shows a plan view of the display system 1. Separate left andright displays (15L, 15R), each with their own imaging optics (17L, 17R)are housed in the central portion (4). These constitute separate lightengines 13L, 13R of the kind just described. Beams created by the leftimaging optics (17L, respective right imaging optics 17R) from a leftimage on the left display (15L, respective right image on the rightdisplay 15R) are coupled into the left optical component (10L,respective right optical component 10R). The beams of the left image(respective right image) are guided though the left component (10L,respective right component 10R) and onto the user's left (respectiveright eye). The guiding mechanism is described in more detail below(note that description pertaining the display/collimating optics 15/17applies equally to both the left display/optics 15L/17L and to the rightdisplay 15R/17R). The left and right images may be different to oneanother in a manner such that a stereoscopic image is perceived by thewearer, i.e. to create an illusion of depth. The left display (15L) andassociated collimating optics (17L) (respective right display 15R andassociated collimating optics 17R) constitute a set of left imagingcomponents (respective right imaging components).

The wearer's ears are not shown in FIG. 2B, however as will be apparent,parts (90L, 90R) of the left and right extensions (6L, 6R) fit over andare supported by the wearer's left and right ears respectively so thatthe optical components (10L, 10R) are supported forward of the user'sleft and right eyes respectively in the manner of conventional spectaclelenses, with the central portion (4) fitting over the nose bridge of thewearer.

Other headpieces are also within the scope of the subject matter. Forinstance, the display optics can equally be attached to the users headusing a head band, helmet or other fit system. The purpose of the fitsystem is to support the display and provide stability to the displayand other head borne systems such as tracking systems and cameras. Thefit system will also be designed to meet user population inanthropometric range and head morphology and provide comfortable supportof the display system. The light engines 17L, 17R may be mounted to acentral portion of any such headpiece so that they sit centrallyrelative to the user when the headpiece is worn, and not at the user'stemples.

Known types of head-mounted display systems tend to locate imagingcomponents to the side of the frame so that they sit near to the user'stemple. This is thought to improve the wearability of the device as thisis generally seen to be the least obtrusive location.

However, the inventors have recognized that, for a stereoscopic imagingsystem, misalignment of a stereoscopic image pair can occur with evenslight changes in the relative position of the left and right opticalimaging components. Such changes can arise from transient deflection ofthe frame through normal use as a result of mechanical or thermaleffects, long term deflection though wear and tear, or other reasonscausing misalignment. Even slight changes can introduce a level ofbinocular disparity between the left and right images to which the humanvisual system (HVS) is highly sensitive, to the extent that evenrelatively short-term exposure to even a small level of binoculardisparity can make the wearer feel quite unwell. The HVS is particularsensitive to vertical disparity between the left and right images, andeven a misalignment of the images by an amount corresponding to aslittle one pixel can be perceptible depending on the display resolution.

The inventors have recognized that in systems, where the left and rightimaging components are located far away from each other, on the sides ofthe frames, maintaining this level of angular alignment between the leftand right components would be impracticable. One way this could beachieved in theory is to make the portion of the frame between the leftand right components sufficiently rigid. However, in practice it isunlikely that the necessary tolerances to maintain binocular paritycould be held, and in any event including any such structure in thesystem would significantly increase manufacturing costs.

The inventors have recognized that were the left and right imagingcomponents to be located to the left and right of the display systemmaintaining this level of angular alignment between the left and rightcomponents would be impracticable. One way this could be achieved, intheory, is to make the portion of the frame between the left and rightcomponents sufficiently rigid. However, in practice it is unlikely thatthe necessary tolerances to maintain binocular parity could be held, andin any event including any such structure in the system wouldsignificantly increase manufacturing costs.

In the display system disclosed herein, the left and right displays arehoused adjacent one another in the central portion (4) of the frame (6).The central portion (4) forms a housing, which houses both of thedisplays (15L, 15R) as well as their respective associated collimatingoptics (17L, 17R).

Collocating both the left and right imaging component (15L/17L, 15R/17R)in this manner ensures that any thermal disturbances affect both thefirst and second images equally and in the same manner (which isacceptable as binocular disparity only results if they are perturbeddifferently to one another). Thus, collocating the left and rightcomponents (15L/17L, 15R/17R) substantially eliminates any binoculardisparity which would otherwise occur due to thermal fluctuations, withthe centrality of the location ensuring each is able to cooperate asintended with the respective optical component (10L, 10R).

Collocating the imaging components (15L/17L, 15R/17R) also means thatmechanical perturbations are less likely to introduce disparity, e.g.twisting or bending of the frame (6) is less likely to introducedisparity when these components are centrally located as compared withlocating them at the sides of the frame.

Although not shown explicitly in FIG. 2B, the imaging component(15L/17L, 15R/17R) are supported in the central portion (4) in a rigidformation by a rigid support structure, for example a carbon fibresupport structure, which is significantly more rigid than the frame (6).Carbon fibre is just an example and other low mass rigid materials couldbe used, e.g. titanium. Supporting both the left and right imagingcomponent in the same highly rigid structure maintains a preciserelative alignment between the left imaging components (15L/17L) and theright imaging components (15R/17R) even in the presence of significantmechanical perturbations. Even if the imaging components move relativeto the frame (6) and in particular relative to the optical components(10L, 10R), binocular parity is maintained because rigidity of thesupport structure keeps the imaging components (15L/17L) and (15R/17R)in a substantially fixed arrangement relative to one another.

Because the left and right imaging components (15L/17L) and (15R/17R)are all located near to one another, the rigid support structure can besmall in size, i.e. requiring a significantly smaller amount of rigidmaterial that if the left and right imaging components were to belocated at the sides of the frame instead. This significantly reducesthe cost of manufacturing the display system.

FIGS. 3A and 3B show an optical component in more detail.

FIG. 3A shows a perspective view of a waveguide optical component (10).The optical component is flat in that the front and rear portions of itssurface are substantially flat (front and rear defined from theviewpoint of the wearer, as indicated by the location of the eye in FIG.3A). The front and rear portions of the surface are parallel to oneanother. The optical component (10) lies substantially in a plane(xy-plane), with the z axis (referred to as the “normal”) directedtowards the viewer from the optical component (10). The incoupling, foldand exit zones (12, 14 and 16) are shown, each defined by respectivesurface modulations (52, 46 and 56) on the surface of the opticalcomponent, which are on the rear of the waveguide from a viewpoint ofthe wearer. Each of the surface modulations (52, 46, 56) forms arespective surface relief grating (SRG), the nature of which will bedescribed shortly. Instead of the SRGs, holograms could be usedproviding the same optical function as the SRGs.

As shown in the plan view of FIG. 3B, the fold zone has a horizontalextent (W2) (referred to herein as the “width” of the expansion zone) inthe lateral (x) direction and an extent (H2) in the vertical (y)direction (referred to herein as the “height” of the expansion zone)which increases from the inner edge of the optical component to itsouter edge in the lateral direction along its width (W2). The exit zonehas a horizontal extent (W3) (width of the exit zone) and y-directionextent (H3) (height of the exit zone) which define the size of the eyebox. The eyebox's size is independent of the imaging optics in the lightengine. The incoupling and fold SRGs (52, 54) have a relativeorientation angle A, as do the fold and exit SRGs (54, 56) (note thevarious dotted lines superimposed on the SRGs 52, 54, 56 in FIG. 9Bdescribed below denote directions perpendicular to the grating lines ofthose SRGs).

The incoupling and fold zones (12, 14) are substantially contiguous inthat they are separated by at most a narrow border zone (18) which has awidth (W) as measured along (that is, perpendicular to) a common border(19) that divides the border zone (18). The common border (19) isarcuate (substantially semi-circular in this example), the incouplingand fold regions (12, 14) having edges which are arcuate (substantiallysemi-circular) along the common border (19). The edge of the incouplingregion (12) is substantially circular overall.

Principles of the diffraction mechanisms which underlie operation of thehead mounted display described herein will now be described withreference to FIGS. 4A and 4B.

The optical components described herein interact with light by way ofreflection, refraction and diffraction. Diffraction occurs when apropagating wave interacts with a structure, such as an obstacle orslit. Diffraction can be described as the interference of waves and ismost pronounced when that structure is comparable in size to thewavelength of the wave. Optical diffraction of visible light is due tothe wave nature of light and can be described as the interference oflight waves. Visible light has wavelengths between approximately 390 and700 nanometres (nm) and diffraction of visible light is most pronouncedwhen propagating light encounters structures of a similar scale e.g. oforder 100 or 1000 nm in scale.

One example of a diffractive structure is a periodic (substantiallyrepeating) diffractive structure. Herein, a “diffraction grating” meansany (part of) an optical component which has a periodic diffractivestructure. Periodic structures can cause diffraction of light, which istypically most pronounced when the periodic structure has a spatialperiod of similar size to the wavelength of the light. Types of periodicstructures include, for instance, surface modulations on the surface ofan optical component, refractive index modulations, holograms etc. Whenpropagating light encounters the periodic structure, diffraction causesthe light to be split into multiple beams in different directions. Thesedirections depend on the wavelength of the light thus diffractionsgratings cause dispersion of polychromatic (e.g. white) light, wherebythe polychromatic light is split into different coloured beamstravelling in different directions.

When the periodic structure is on the surface of an optical component,it is referred to a surface grating. When the periodic structure is dueto modulation of the surface itself, it is referred to as a surfacerelief grating (SRG). An example of a SRG is uniform straight grooves ina surface of an optical component that are separated by uniform straightgroove spacing regions. Groove spacing regions are referred to herein as“lines”, “grating lines” and “filling regions”. The nature of thediffraction by a SRG depends both on the wavelength of light incident onthe grating and various optical characteristics of the SRG, such as linespacing, groove depth and groove slant angle. An SRG can be fabricatedby way of a suitable microfabrication process, which may involve etchingof and/or deposition on a substrate to fabricate a desired periodicmicrostructure on the substrate to form an optical component, which maythen be used as a production master such as a mould for manufacturingfurther optical components.

An SRG is an example of a Diffractive Optical Element (DOE). When a DOEis present on a surface (e.g. when the DOE is an SRG), the portion ofthat surface spanned by that DOE is referred to as a DOE area.

FIGS. 4A and 4B show from the top and the side respectively part of asubstantially transparent optical component (10) having an outer surface(S). At least a portion of the surface S exhibits surface modulationsthat constitute a SRG (44) (e.g. 52, 54, 56), which is a microstructure.Such a portion is referred to as a “grating area”. The modulationscomprise grating lines which are substantially parallel and elongate(substantially longer than they are wide), and also substantiallystraight in this example (though they need not be straight in general).

FIG. 4B shows the optical component (10), and in particular the SRG(44), interacting with an incoming illuminating light beam I that isinwardly incident on the SRG (44). The incident light (I) is white lightin this example, and thus has multiple colour components. The light (I)interacts with the SRG (44) which splits the light into several beamsdirected inwardly into the optical component (10). Some of the light (I)may also be reflected back from the surface (S) as a reflected beam(RO). A zero-order mode inward beam (T0) and any reflection (R0) arecreated in accordance with the normal principles of diffraction as wellas other non-zero-order (±n-order) modes (which can be explained as waveinterference). FIG. 4B shows first-order inward beams (T1, T-1); it willbe appreciated that higher-order beams may or may not also be createddepending on the configuration of the optical component (10). Becausethe nature of the diffraction is dependent on wavelength, forhigher-order modes, different colour components (i.e. wavelengthcomponents) of the incident light (I) are, when present, split intobeams of different colours at different angles of propagation relativeto one another as illustrated in FIG. 4B.

FIGS. 5A-5C are close-up schematic cross sectional views of differentexemplary SRGs 44 a-44 c (collectively referenced as 44 herein) that maybe formed by modulation of the surface S of the optical component 10(which is viewed from the side in these figures). Light beams aredenoted as arrows whose thicknesses denote approximate relativeintensity (with higher intensity beams shown as thicker arrows).

FIG. 5A shows an example of a straight binary SRG (44 a). The straightbinary SRG (44 a) is formed of a series of grooves (7 a) in the surface(S) separated by protruding groove spacing regions (9 a) which are alsoreferred to herein as “filling regions”, “grating lines” or simply“lines”. The SRG (44 a) has a spatial period of d (referred to as the“grating period”), which is the distance over which the modulations'shape repeats and which is thus the distance between adjacentlines/grooves. The grooves (7 a) have a depth (h) and have substantiallystraight walls and substantially flat bases. The filling regions have aheight (h) and a width that is substantially uniform over the height (h)of the filling regions, labelled “w” in FIG. 5A (with w being somefraction f of the period: w=f*d).

For a straight binary SRG, the walls are substantially perpendicular tothe surface (S). For this reason, the SRG (44 a) causes symmetricdiffraction of incident light (I) that is entering perpendicularly tothe surface, in that each +n-order mode beam (e.g. T1) created by theSRG (4 a) has substantially the same intensity as the corresponding-n-order mode beam (e.g. T-1), typically less than about one fifth (0.2)of the intensity of the incident beam (I).

FIG. 5B shows an example of a slanted binary SRG (44 b). The slantedbinary SRG (44 b) is also formed of grooves, labelled 7 b, in thesurface (S) having substantially straight walls and substantially flatbases separated by lines (9 b) of width (w). However, in contrast to thestraight SRG (44 a), the walls are slanted by an amount relative to thenormal, denoted by the angle β in FIG. 5B. The grooves (7 b) have adepth (h) as measured along the normal. Due to the asymmetry introducedby the non-zero slant, ±n-order mode inward beams travelling away fromthe slant direction have greater intensity that their ±n-order modecounterparts (e.g. in the example of FIG. 5B, the T1 beam is directedaway from the direction of slant and has usually greater intensity thanthe T-1 beam, though this depends on e.g. the grating period d); byincreasing the slant by a sufficient amount, those ±n counterparts canbe substantially eliminated (i.e. to have substantially zero intensity).The intensity of the T0 beam is typically also very much reduced by aslanted binary SRG such that, in the example of FIG. 5B, the first-orderbeam T1 typically has an intensity of at most about four fifths (0.8)the intensity of the incident beam (I).

The binary SRGs (44 a) and (44 b) can be viewed as spatial waveformsembedded in the surface (S) that have a substantially square wave shape(with period d). In the case of the SRG (44 b), the shape is a skewedsquare wave shape skewed by β.

FIG. 5C shows an example of an overhanging triangular SRG (44 c) whichis a special case of an overhanging trapezoidal SRG. The triangular SRG(44 c) is formed of grooves (7 c) in the surface (S) that are triangularin shape (and which thus have discernible tips) and which have a depth(h) as measured along the normal. Filling regions (9 c) take the form oftriangular, tooth-like protrusions (teeth), having medians that make anangle β with the normal (β being the slant angle of the SRG 44 c). Theteeth have tips that are separated by (d) (which is the grating periodof the SRG 44 c), a width that is (w) at the base of the teeth and whichnarrows to substantially zero at the tips of the teeth. For the SRG (44c) of FIG. 5C, w≈d, but generally can be w<d. The SRG is overhanging inthat the tips of the teeth extend over the tips of the grooves. It ispossible to construct overhanging triangular SRGs that substantiallyeliminate both the zero order transmission-mode (T0) beam and the±n-mode beams, leaving only ±n-order mode beams (e.g. only T1). Thegrooves have walls which are at an angle γ to the median (wall angle).

The SRG (44 c) can be viewed as a spatial waveform embedded in (S) thathas a substantially triangular wave shape, which is skewed by β.

Other SRGs are also possible, for example other types of trapezoidalSRGs (which may not narrow in width all the way to zero), sinusoidalSRGs etc. Such other SRGs also exhibit depth (h), linewidth (w), slantangle β and wall angles γ which can be defined in a similar manner toFIG. 5A-C.

In the present display system, d is typically between about 250 and 500nm, and h between about 30 and 400 nm. The slant angle β is typicallybetween about 0 and 45 degrees (such that slant direction is typicallyelevated above the surface (S) by an amount between about 45 and 90degrees).

An SRG has a diffraction efficiency defined in terms of the intensity ofdesired diffracted beam(s) (e.g. T1) relative to the intensity of theilluminating beam (I), and can be expressed as a ratio (η) of thoseintensities. As will be apparent from the above, slanted binary SRGs canachieve higher efficiency (e.g. 4 b—up to η≈0.8 if T1 is the desiredbeam) than non-slanted SRGs (e.g. 44 a—only up to about η≈0.2 if T1 isthe desired beam). With overhanging triangular SRGs, it is possible toachieve near-optimal efficiencies of η≈1.

Returning to FIGS. 3A and 3B, it can be seen that the incoupling, foldand exit zones (12, 14, 16) are diffraction gratings whose periodicstructure arises due to the modulations (52, 54, 56) of the opticalcomponent's surface that form the incoupling, fold and exit SRGsrespectively, and which cover the incoupling, fold and exit zones 12,14, 16 respectively.

FIG. 6 shows the incoupling SRG (52) with greater clarity, including anexpanded version showing how the light beam interacts with it. FIG. 6shows a plan view of the optical component (10). The light engine (13)provides beams of collimated light, one of which is shown (correspondingto a display pixel). That beam falls on the incoupling SRG (52) and thuscauses total internal reflection of the beam in the component (10). Theintermediate grating (14) directs versions of the beams down to the exitgrating (16), which causes diffraction of the image onto the user's eye.The operation of the grating (12) is shown in more detail in theexpanded portion which shows rays of the incoming light beam coming infrom the left and denoted (I) and those rays being diffracted so as toundergo TIR in the optical component (10). The grating in FIG. 6 is ofthe type shown in FIG. 5B but could also be of the type shown in FIG. 5Cor some other slanted grating shape.

Optical principles underlying certain embodiments will now be describedwith reference to FIGS. 7A-9B.

Collimating optics of the display system are arranged to substantiallycollimate an image on a display of the display system into multipleinput beams. Each beam is formed by collimating light from a respectiveimage point, that beam directed to the incoupling zone in a uniqueinward direction which depends on the location of that point in theimage. The multiple input beams thus form a virtual version of theimage. The intermediate and exit zones have widths substantially largerthan the beams' diameters. The incoupling zone is arranged to coupleeach beam into the intermediate zone, in which that beam is guided ontomultiple splitting regions of the intermediate zone in a direction alongthe width of the intermediate zone. The intermediate zone is arranged tosplit that beam at the splitting regions to provide multiplesubstantially parallel versions of that beam. Those multiple versionsare coupled into the exit zone, in which the multiple versions areguided onto multiple exit regions of the exit zone. The exit regions liein a direction along the width of the exit zone. The exit zone isarranged to diffract the multiple versions of that beam outwardly,substantially in parallel and in an outward direction whichsubstantially matches the unique inward direction in which that beam wasincoupled. The multiple input beams thus cause multiple exit beams toexit the waveguide which form substantially the same virtual version ofthe image.

FIG. 7a shows a perspective view of the display (15), imaging optics(17) and incoupling SRG (52). Different geometric points on the regionof the display (15) on which an image is displayed are referred toherein as image points, which may be active (currently emitting light)or inactive (not currently emitting light). In practice, individualpixels can be approximated as image points.

The imaging optics (17) can typically be approximated as a principalplane (thin lens approximation) or, in some cases, more accurately as apair of principal planes (thick lens approximation) the location(s) ofwhich are determined by the nature and arrangement of its constituentlenses. In these approximations, any refraction caused by the imagingoptics (17) is approximated as occurring at the principal plane(s). Toavoid unnecessary complication, principles of various embodiments willbe described in relation to a thin lens approximation of the imagingoptics (17), and thus in relation to a single principal plane labelled31 in FIG. 7a , but it will be apparent that more complex imaging opticsthat do not fit this approximation still can be utilized to achieve thedesired effects.

The imaging optics (17) has an optical axis (30) and a front focalpoint, and is positioned relative to the optical component (10) so thatthe optical axis (30) intersects the incoupling SRG (52) at or near thegeometric centre of the incoupling SRG (52) with the front focal pointlying substantially at an image point X₀ on the display (that is, lyingin the same plane as the front of the display). Another arbitrary imagepoint X on the display is shown, and principles underlying variousembodiments will now be described in relation to X without loss ofgenerality. In the following, the terminology “for each X” or similar isused as a convenient shorthand to mean “for each image point (includingX)” or similar, as will be apparent in context.

When active, image points—including the image point labelled X andX₀—act as individual illumination point sources from which lightpropagates in a substantially isotropic manner through the half-spaceforward of the display (15). Image points in areas of the imageperceived as lighter emit light of higher intensity relative to areas ofthe image perceived as darker. Image points in areas perceived as blackemit no or only very low intensity light (inactive image points). Theintensity of the light emitted by a particular image point may change asthe image changes, for instance when a video is displayed on the display(15).

Each active image point provides substantially uniform illumination of acollimating area (A) of the imaging optics (17), which is substantiallycircular and has a diameter (D) that depends on factors such as thediameters of the constituent lenses (typically D is of order 1-10 mm).This is illustrated for the image point X in FIG. 7a , which shows howany propagating light within a cone 32(X) from X is incident on thecollimating area A. The imaging optics collimates any light 32(X)incident on the collimating area A to form a collimated beam 34(X) ofdiameter D (input beam), which is directed towards the incouplinggrating (52) of the optical component (10). The beam 34(X) is thusincident on the incoupling grating (52). A shielding component (notshown) may be arranged to prevent any un-collimated light from outsideof the cone 32(X) that is emitted from X from reaching the opticalcomponent (10).

The beam 34(X) corresponding to the image point X is directed in aninward propagation direction towards the incoupling SRG (52), which canbe described by a propagation vector {circumflex over (k)}_(in)(X)(herein, bold typeface is used to denote 3-dimensional vectors, withhats on such vectors indicating denoting a unit vector). The inwardpropagation direction depends on the location of X in the image and,moreover, is unique to X. That unique propagation direction can beparameterized in terms of an azimuthal angle ϕ_(in)(X) (which is theangle between the x-axis and the projection of {circumflex over(k)}_(in)(X) in the xy-plane) and a polar angle θ_(in)(X) (which is theangle between the z-axis and {circumflex over (k)}_(in)(P) as measuredin the plane in which both the z-axis and {circumflex over (k)}_(in)(X)lie—note this is not the xz-plane in general). The notation ϕ_(in)(X),θ_(in)(X) is adopted to denote the aforementioned dependence on X; asindicated ϕ_(in)(X), θ_(in)(X) are unique to that X. Note that, herein,both such unit vectors and such polar/azimuthal angle pairsparameterizing such vectors are sometimes referred herein to as“directions” (as the latter represent complete parameterizationsthereof), and that sometimes azimuthal angles are referred to inisolation as xy-directions for the same reason. Note further that“inward” is used herein to refer to propagation that is towards thewaveguide (having a positive z-component when propagation is towards therear of the waveguide as perceived by the viewer and a negativez-component when propagation is towards the front of the waveguide).

The imaging optics has a principle point P, which is the point at whichthe optical axis (30) intersects the principal plane (31) and whichtypically lies at or near the centre of the collimation area (A). Theinward direction {circumflex over (k)}_(in)(X) and the optical axis 30have an angular separation β(X) equal to the angle subtended by X and X₀from P. β(X)=θ_(in)(X) if the optical axis is parallel to the z-axis(which is not necessarily the case).

As will be apparent, the above applies for each active image point andthe imaging optics is thus arranged to substantially collimate theimage, which is currently on the display (15), into multiple inputbeams, each corresponding to and propagating in a unique directiondetermined by the location of a respective active image point (activepixel in practice). That is, the imaging optics (17) effectivelyconverts each active point source (X) into a collimated beam in a uniqueinward direction {circumflex over (k)}_(in)(X). As will be apparent,this can be equivalently stated as the various input beams for all theactive image points forming a virtual image at infinity that correspondsto the real image that is currently on the display (15). A virtual imageof this nature is sometimes referred to herein as a virtual version ofthe image (or similar).

The input beam corresponding to the image point X₀ (not shown) wouldpropagate parallel to the optical axis (30), towards or near thegeometric centre of the incoupling SRG (52).

As mentioned, in practice, individual pixels of the display (15) can beapproximated as single image points. This is illustrated in FIG. 7Bwhich is a schematic plan view showing the principal plane (31) and twoadjacent pixels (Xa, Xb) of the display (15), whose centres subtend anangle Δβ from the principal point P. Light emitted the pixels (Xa, Xb)when active is effectively converted into collimated beams 34(Xa),34(Xb) having an angular separation equal to Δβ. As will be apparent,the scale of the pixels (Xa, Xb) has been greatly enlarged for thepurposes of illustration.

The beams are highly collimated, having an angular range no greater thanthe angle subtended by an individual pixel from P (˜Δβ), e.g. typicallyhaving an angular range no more than about ½ milliradian. As will becomeapparent in view of the following, this increases the image quality ofthe final image as perceived by the wearer.

FIGS. 7C and 7D show schematic plan (xz) and frontal (yz) views of partof the optical component respectively. As indicated in these figures,the incoupling grating (52) causes diffraction of the beam 34(X) therebycausing a first (±1) order mode beam to propagate within the opticalcomponent (10) in a new direction {circumflex over (k)}(X) that isgenerally towards the fold SRG (54) (i.e. that has a positivex-component). The new direction {circumflex over (k)}(X) can beparameterized by azimuthal and polar angles ϕ(X)—where|ϕ(X)|≤|ϕ_(in)(X)| and θ(X)—where |θ(X)|>|θ_(in)(X)|—which are alsodetermined by the location of and unique to the image point X. Thegrating (52) is configured so that the first order mode is the onlysignificant diffraction mode, with the intensity of this new beam thussubstantially matching that of the input beam. As mentioned above, aslanted grating can be used to achieve this desired effect (the beam asdirected away from the incoupling SRG (52) would correspond, forinstance, to beam T1 as shown in FIGS. 4B or 4C). In this manner, thebeam 34(X) is coupled into the incoupling zone (12) of the opticalcomponent (10) in the new direction {circumflex over (k)}(X).

The optical component has a refractive index n and is configured suchthat the polar angle θ(X) satisfies total internal reflection criteriagiven by:sin θ(X)>1/n for each X.  (1)As will be apparent, each beam input from the imaging optics (17) thuspropagates through the optical component (10) by way of total internalreflection (TIR) in a generally horizontal (+x) direction (offset fromthe x-axis by ϕ(X)<ϕ_(in)(X)). In this manner, the beam 34(X) is coupledfrom the incoupling zone (12) into the fold zone (14), in which itpropagates along the width of the fold zone (14).

FIG. 7E shows a frontal (xy) view of the whole of the optical component(10), from a viewpoint similar to that of the wearer. As explained inmore detail below, a combination of diffractive beam splitting and totalinternal reflection within the optical component (10) results inmultiple versions of each input beam 34(X) being outwardly diffractedfrom the exit SRG along both the length and the width of the exit zone(16) as output beams 38(X) in respective outward directions (that is,away from the optical component 10) that substantially match therespective inward direction {circumflex over (k)}_(in)(X) of thecorresponding input beam 34(X).

In FIG. 7E, beams external to the optical component (10) are representedusing shading and dotted lines are used to represent beams within theoptical component 10. Perspective is used to indicate propagation in thez-direction, with widening (resp. narrowing) of the beams in FIG. 7Erepresenting propagation in the positive (resp. negative) z direction;that is towards (resp. away from) the wearer. Thus, diverging dottedlines represent beams within the optical component (10) propagatingtowards the front wall of the optical component (10); the widest partsrepresent those beams striking the front wall of the optical component10, from which they are totally internally reflected back towards therear wall (on which the various SRGs are formed), which is representedby the dotted lines converging from the widest points to the narrowestpoints at which they are incident on the rear wall. Regions where thevarious beams are incident on the fold and exit SRGs are labelled S andE and termed splitting and exit regions respectively for reasons thatwill become apparent.

As illustrated, the input beam 34(X) is coupled into the waveguide byway of the aforementioned diffraction by the incoupling SRG (52), andpropagates along the width of the incoupling zone (12) by way of TIR inthe direction ϕ(X), ±θ(X) (the sign but not the magnitude of the polarangle changing whenever the beam is reflected). As will be apparent,this results in the beam 34(X) eventually striking the fold SRG at theleft-most splitting region (S).

When the beam 34(X) is incident at a splitting region (S,) that incidentbeam 34(X) is effectively split in two by way of diffraction to create anew version of that beam 42(X) (specifically a −1 reflection mode beam)which directed in a specific and generally downwards (−y) directionϕ′(X), ±θ′(X) towards the exit zone (16) due to the fold SRG (54) havinga particular configuration which will be described in due course, inaddition to a zero order reflection mode beam (specular reflectionbeam), which continues to propagate along the width of the beam in thesame direction ϕ(X), ±θ(X) just as the beam 34(X) would in the absenceof the fold SRG (albeit at a reduced intensity). Thus, the beam 34(X)effectively continues to propagate along substantially the whole widthof the fold zone (14), striking the fold SRG at various splittingregions (S), with another new version of the beam (in the same specificdownward direction ϕ′(X), ±θ′(X)) created at each splitting region (S).As shown in FIG. 7E, this results in multiple versions of the beam 34(X)being coupled into the exit zone (16), which are horizontally separatedso as to collectively span substantially the width of the exit zone(16).

As also shown in FIG. 7E, a new version 42(X) of the beam as created ata splitting region (S) may itself strike the fold SRG during itsdownward propagation. This will result in a zero order mode beingcreated which continues to propagate generally downwards in thedirection ϕ′(X), ±θ′(X) and which can be viewed as continued propagationof that beam, but may also result in a non-zero order mode beam 40(X)(further new version) being created by way of diffraction. However, anysuch beam 40(X) created by way of such double diffraction at the sameSRG will propagate in substantially the same direction ϕ(X), ±θ(X) alongthe width of the fold zone (14) as the original beam 34(X) as coupledinto the optical component (10) (see below). Thus, notwithstanding thepossibility of multiple diffractions by the fold SRG, propagation of thevarious versions of the beam 34(X) (corresponding to image point X)within the optical component (10) is effectively limited to twoxy-directions: the generally horizontal direction (ϕ(X), ±θ(X)), and thespecific and generally downward direction (ϕ′(X), ±θ′(X)) that will bediscussed shortly.

Propagation within the fold zone (14) is thus highly regular, with allbeam versions corresponding to a particular image point X substantiallyconstrained to a lattice like structure in the manner illustrated.

The exit zone (16) is located below the fold zone (14) and thus thedownward-propagating versions of the beam 42(X) are coupled into theexit zone (16), in which they are guided onto the various exit regions(E) of the output SRG. The exit SRG (56) is configured so as, when aversion of the beam strikes the output SRG, that beam is diffracted tocreate a first order mode beam directed outwardly from the exit SRG (56)in an outward direction that substantially matches the unique inwarddirection in which the original beam 34(X) corresponding to image pointX was inputted. Because there are multiple versions of the beampropagating downwards that are substantially span the width of the exitzone (16), multiple output beams are generated across the width of theexit zone (16) (as shown in FIG. 7E) to provide effective horizontalbeam expansion.

Moreover, the exit SRG (56) is configured so that, in addition to theoutwardly diffracted beams 38(X) being created at the various exitregions (E) from an incident beam, a zero order diffraction mode beamcontinuous to propagate downwards in the same specific direction as thatincident beam. This, in turn, strikes the exit SRG at a lower exit zone(16) in the manner illustrated in FIG. 7E, resulting in both continuingzero-order and outward first order beams. Thus, multiple output beams38(X) are also generated across substantially the width of the exit zone(16) to provide effective vertical beam expansion.

The output beams 38(X) are directed outwardly in outward directions thatsubstantially match the unique input direction in which the originalbeam 34(X) is inputted. In this context, substantially matching meansthat the outward direction is related to the input direction in a mannerthat enables the wearer's eye to focus any combination of the outputbeams 38(X) to a single point on the retina, thus reconstructing theimage point X (see below).

For a flat optical component (that is, whose front and rear surfaces liesubstantially parallel to the xy-plane in their entirety), the outputbeams are substantially parallel to one another (to at least within theangle Δβ subtended by two adjacent display pixels) and propagateoutwardly in an output propagation direction {circumflex over(k)}_(out)(X) that is parallel to the unique inward direction{circumflex over (k)}_(in)(X) in which the corresponding input beam34(X) was directed to the incoupling SRG (52). That is, directing thebeam 34(X) corresponding to the image point X to the incoupling SRG (52)in the inward direction {circumflex over (k)}_(in)(X) causescorresponding output beams 38(X) to be diffracted outwardly and inparallel from the exit zone (16), each in an outward propagationdirection {circumflex over (k)}_(out)(X)={circumflex over (k)}_(in)(X)due to the configuration of the various SRGs (see below).

As will now be described with reference to FIG. 7F, this enables aviewer's eye to reconstruct the image when looking at the exit zone(16). FIG. 7F shows a plan (xz) view of the optical component 10. Theinput beam 34(X) is in coupled to the optical component (10) resultingin multiple parallel output beams 38(X) being created at the variousexit regions (E) in the manner discussed above. This can be equivalentlyexpressed at the various output beams corresponding to all the imagepoints forming the same virtual image (at infinity) as the correspondinginput beams.

Because the beams 38(X) corresponding to the image point X are allsubstantially parallel, any light of one or more of the beam(s) 38(X)which is received by the eye (37) is focused as if the eye (37) wereperceiving an image at infinity (i.e. a distant image). The eye (37)thus focuses such received light onto a single retina point, just as ifthe light were being received from the imaging optics (17) directly,thus reconstructing the image point X (e.g. pixel) on the retina. Aswill be apparent, the same is true of each active image point (e.g.pixel) so that the eye (37) reconstructs the whole image that iscurrently on the display (15).

However, in contrast to receiving the image directly from the optics(17)—from which only a respective single beam 34(X) of diameter D isemitted for each X—the output beams 38(X) are emitted over asignificantly wider area i.e. substantially that of the exit zone (16),which is substantially larger than the area of the inputted beam (˜D²).It does not matter which (parts) of the beam(s) 38(X) the eye receivesas all are focused to the same retina point—e.g., were the eye (37) tobe moved horizontally (±x) in FIG. 7F, it is apparent that the imagewill still be perceived. Thus, no adaptation of the display system isrequired for, viewers with different pupillary distances beyond makingthe exit zone (16) wide enough to anticipate a reasonable range ofpupillary distances, whilst viewers whose eyes are closer together willgenerally receive light from the side of the exit zone (16) nearer theincoupling zone (12) as compared with viewers whose eyes are furtherapart, both will nonetheless perceive the same image. Moreover, as theeye (37) rotates, different parts of the image are brought towards thecentre of the viewer's field of vision (as the angle of the beamsrelative to the optical axis of the eye changes) with the image stillremaining visible, thereby allowing the viewer to focus their attentionon different parts of the image as desired.

The same relative angular separation Δβ exhibited the input beamscorresponding any two adjacent pixels (Xa, Xb) is also exhibited by thecorresponding sets of output beams 38(Xa), 38(Xb)—thus adjacent pixelsare focused to adjacent retina points by the eye (37). All the variousversions of the beam remain highly collimated as they propagate throughthe optical component (10), preventing significant overlap of pixelimages as focused on the retina, thereby preserving image sharpness.

It should be noted that FIGS. 7A-7G are not to scale and that inparticular beams diameters are, for the sake of clarity, generallyreduced relative to components such as the display (15) than wouldtypically be expected in practice.

The configuration of the incoupling SRG (52) will now be described withreference to FIGS. 8A and 8B, which show schematic plan and frontalviews of part of the fold grating (52). Note, in FIGS. 8A and 8B, beamsare represented by arrows (that is, their area is not represented) forthe sake of clarity.

FIG. 8A shows two image points (XL, XR) located at the far left and farright of the display (15) respectively, from which light is collimatedby the optics (17) to generate respective input beams 34(XL), 34(XR) ininward directions (θ_(in)(XL), ϕ_(in)(XL)), (θ_(in)(XR), ϕ_(in)(XR)).These beams are coupled into the optical component (10) by theincoupling SRG (52) as shown—the incoupled beams shown created at theincoupling SRG (52) are first order (+1) mode beams created by way ofdiffraction of the beams incident on the SRG (52). The beams 34(XL),34(XR) as coupled into the waveguide propagate in directions defined bythe polar angles θ(XL), θ(XR).

FIG. 8B shows two image points XR1 and XR2 at the far top-right and farbottom-right of the display (15). Note in this figure dashed-dottedlines denote aspects which are behind the optical component (10) (−z).Corresponding beams 34(XL), 34(XR) in directions within the opticalcomponent (10) with polar angles ϕ(XL), ϕ(XR). Such angles θ(X), ϕ(X)are given by the (transmissive) grating equations:

$\begin{matrix}{{n\mspace{14mu}\sin\mspace{11mu}{\theta(X)}\mspace{11mu}\sin\mspace{11mu}{\phi(X)}} = {\sin\mspace{11mu}{\theta_{in}(X)}\mspace{11mu}\sin\mspace{11mu}{\phi_{in}(X)}}} & (2) \\{{n\mspace{14mu}\sin\mspace{11mu}{\theta(X)}\mspace{11mu}\cos\mspace{11mu}{\phi(X)}} = {{\sin\mspace{11mu}{\theta_{in}(X)}\mspace{11mu}\cos\mspace{11mu}{\phi_{in}(X)}} + \frac{\lambda}{d_{1}}}} & (3)\end{matrix}$with the SRG (52) having a grating period d₁, the beam light having awavelength λ, and n the refractive index of the optical component.

It is straightforward to show from equations (2), (3) that θ(XL)=θ_(max)and θ(XR)=θ_(min) i.e. that any beam as coupled into the component (10)propagates with an initial polar angle in the range [θ(XR), θ(XL)]; andthat ϕ(XR2)=ϕ_(max) and ϕ(XR1)=ϕ_(min)(≈−ϕ_(max) in this example) i.e.that any beam as coupled into the component initially propagates with anazimuthal angle in the range [ϕ(XR1), ϕ(XR2)](≈[−ϕ(XR2), ϕ(XR2)]).

The configuration of the fold SRG (54) will now be described withreferences to FIGS. 9A-9B. Note, in FIGS. 9A and 9B, beams are againrepresented by arrows, without any representation of their areas, forthe sake of clarity. In these figures, dotted lines denote orientationsperpendicular to the fold SRG grating lines, dashed lines orientationsperpendicular to the incoupling SRG grating lines, and dash-dotted linesorientations perpendicular to the exit SRG grating lines.

FIG. 9A shows a perspective view of the beam 34(X) as coupled into thefold zone (14) of the optical component (10), having been reflected fromthe front wall of the optical component (10) and thus travelling in thedirection (ϕ(X), −θ(X)) towards the fold SRG (54). A dotted line (whichlies perpendicular to the fold SRG grating lines) is shown to representthe orientation of the fold SRG.

The fold SRG (54) and incoupling SRG (52) have a relative orientationangle A (which is the angle between their respective grating lines). Thebeam thus makes an angle A+ϕ(X) (see FIG. 9B) with the fold SRG gratinglines as measured in the xy-plane. The beam (34) is incident on the foldSRG (54), which diffracts the beam (34) into different components. Azero order reflection mode (specular reflection) beam is created whichcontinues to propagate in the direction (ϕ(X), +θ(X)) just as the beam34(X) would due to reflection in the absence of the fold SRG (54)(albeit at a reduced intensity). This specular reflection beam can beviewed as effectively a continuation of the beam 34(X) and for thisreason is also labelled 34(X). A first order (−1) reflection mode beam42(X) is also created which can be effectively considered a new versionof the beam.

As indicated, the new version of the beam 42(X) propagates in a specificdirection (ϕ′(X), θ′(X)) which is given by the known (reflective)grating equations:

$\begin{matrix}{{n\mspace{14mu}\sin\mspace{11mu}{\theta^{\prime}(X)}\mspace{11mu}{\sin\left( {A + {\phi^{\prime}(X)}} \right)}} = {n\mspace{14mu}\sin\mspace{11mu}{\theta(X)}\mspace{11mu}{\sin\left( {A + {\phi(X)}} \right)}}} & (4) \\{{n\mspace{14mu}\sin\mspace{11mu}{\theta^{\prime}(X)}\mspace{11mu}{\cos\left( {A + {\phi^{\prime}(X)}} \right)}} = {{n\mspace{14mu}\sin\mspace{11mu}{\theta(X)}\mspace{11mu}{\cos\left( {A + {\phi(X)}} \right)}} - \frac{\lambda}{d_{2}}}} & (5)\end{matrix}$where the fold SRG has a grating period d₂, the beam light has awavelength λ and n is the refractive index of the optical component(10).

As shown in FIG. 9B, which shows a schematic frontal view of the opticalcomponent (10), the beam 34(X) is coupled into the incoupling zone (12)with azimuthal angle ϕ(X) and thus makes an xy-angle ϕ(X)+A the fold SRG54.

A first new version 42 a(X) (−1 mode) of the beam 34(X) is created whenit is first diffracted by the fold SRG (54) and a second new version 42b(X) (−1 mode) when it is next diffracted by the fold SRG 54 (and soon), which both propagate in xy-direction ϕ′(X). In this manner, thebeam 34(X) is effectively split into multiple versions, which arehorizontally separated (across the width of the fold zone 14). These aredirected down towards the exit zone (16) and thus coupled into the exitzone (16) (across substantially the width of the exit zone 16 due to thehorizontal separation). As can be seen, the multiple versions are thusincident on the various exit regions (labelled E) of the exit SRG (56),which lie along the width of the exit zone (16).

These new, downward (−y)-propagating versions may themselves meet thefold SRG (54) once again, as illustrated. However, it can be shown fromequations (4), (5) that any first order reflection mode beam (e.g. 40a(X), +1 mode) created by diffraction at an SRG of an incident beam(e.g. 42 a(X), −1 mode) which is itself a first order reflection modebeam created by an earlier diffraction of an original beam (e.g. 34(X))at the same SRG will revert to the direction of the original beam (e.g.ϕ(X), ±θ(X), which is the direction of propagation of 40 a(X)). Thus,propagation within the fold zone (14) is restricted to a diamond-likelattice, as can be seen from the geometry of FIG. 9B. The beam labelled42 ab(X) is a superposition of a specular reflection beam created when42 b(X) meets the fold SRG (54) and a −1 mode beam created when 40 a(X)meets the fold SRG (54) at substantially the same location; the beamlabelled 42 ab(X) is a superposition of a specular reflection beamcreated when 40 a(X) meets the fold SRG (54) and a +1 mode beam createdwhen 42 b(X) meets the fold SRG at substantially the same location (andso on).

The exit SRG and incoupling SRG (52, 56) are oriented with a relativeorientation angle A′ (which is the angle between their respectivegrating lines). At each of the exit regions, the version meeting thatregion is diffracted so that, in addition to a zero order reflectionmode beam propagating downwards in the direction ϕ′(X), ±θ′(X), a firstorder (+1) transmission mode beam 38(X) which propagates away from theoptical component (10) in an outward direction ϕ_(out)(X), θ_(out)(X)given by:

$\begin{matrix}{\mspace{79mu}{{\sin\mspace{11mu}{\theta_{out}(X)}\mspace{11mu}{\sin\left( {A^{\prime} + {\phi_{out}(X)}} \right)}} = {n\mspace{14mu}\sin\mspace{11mu}{\theta^{\prime}(X)}\mspace{11mu}{\sin\left( {A^{\prime} + {\phi^{\prime}(X)}} \right)}}}} & (6) \\{{\sin\mspace{11mu}{\theta_{out}(X)}\mspace{11mu}{\cos\left( {A^{\prime} + {\phi_{out}(X)}} \right)}} = {{n\mspace{14mu}\sin\mspace{11mu}{\theta^{\prime}(X)}\mspace{11mu}{\cos\left( {A^{\prime} + {\phi^{\prime}(X)}} \right)}} + \frac{\lambda}{d_{3}}}} & (7)\end{matrix}$The output direction θ_(out)(X), ϕ_(out)(X) is that of the output beamsoutside of the waveguide (propagating in air). For a flat waveguide,equations (6), (7) hold both when the exit grating is on the front ofthe waveguide—in which case the output beams are first ordertransmission mode beams (as can be seen, equations (6), (7) correspondto the known transmission grating equations)—but also when the exitgrating is on the rear of the waveguide (as in FIG. 7F)—in which casethe output beams correspond to first order reflection mode beams which,upon initial reflection from the rear exit grating propagate in adirection θ′_(out)(X), ϕ′_(out)(X) within the optical component (10)given by:

$\begin{matrix}{\mspace{79mu}{{n\mspace{14mu}\sin\mspace{11mu}{{\theta^{\prime}}_{out}(X)}\mspace{11mu}{\sin\left( {A^{\prime} + {{\phi^{\prime}}_{out}(X)}} \right)}} = {n\mspace{14mu}\sin\mspace{11mu}{\theta^{\prime}(X)}\mspace{11mu}{\sin\left( {A^{\prime} + {\phi^{\prime}(X)}} \right)}}}} & \left( 6^{\prime} \right) \\{{{n\mspace{14mu}\sin\mspace{11mu}{{\theta^{\prime}}_{out}(X)}\mspace{11mu}{\cos\left( {A^{\prime} + {{\phi^{\prime}}_{out}(X)}} \right)}} = {{n\mspace{14mu}\sin\mspace{11mu}{\theta^{\prime}(X)}\mspace{11mu}{\cos\left( {A^{\prime} + {\phi^{\prime}(X)}} \right)}} + \frac{\lambda}{d_{3}}}};} & \left( 7^{\prime} \right)\end{matrix}$

These beams are then refracted at the front surface of the opticalcomponent, and thus exit the optical component in a direction θ_(in)(X),ϕ_(in)(X) given by Snell's law:sin θ_(out)(X)=n sin θ′_(out)(X)  (8)ϕ′_(out)(X)=ϕ_(out)(X)  (9)As will be apparent, the conditions of equations (6), (7) followstraight forwardly from equations (6′),(7′),(8) and (9). Note that suchrefraction at the front surface, whilst not readily visible in FIG. 7F,will nonetheless occur in the arrangement of FIG. 7F.It can be shown from the equations (2-7) that, whend=d₁=d₃  (10)(that is, when the periods of the incoupling and exit SRGs 52, 56substantially match);d ₂ =d/(2 cos A);  (11)andA′=2A;  (12)then (θ_(out)(X), ϕ_(out)(X))=(θ_(in)(X), ϕ_(in)(X)).Moreover, when the condition

$\begin{matrix}{\sqrt{\left( {1 + {8\mspace{11mu}\cos^{2}\; A}} \right)} > \frac{nd}{\lambda}} & (13)\end{matrix}$is met, no modes besides the above-mentioned first order and zero orderreflection modes are created by diffraction at the fold SRG (54). Thatis, no additional undesired beams are created in the fold zone when thiscriteria is met. The condition in equation (13) is met for a large rangeof A, from about 0 to 70 degrees.

In other words, when these criteria are met, the exit SRG (56)effectively acts as an inverse to the incoupling SRG (52), reversing theeffect of the incoupling SRG diffraction for each version of the beamwith which it interacts, thereby outputting what is effectively atwo-dimensionally expanded version of that beam 34(X) having an areasubstantially that of the exit SRG (56) (>>D² and which, as noted, isindependent of the imaging optics 17) in the same direction as theoriginal beam was inputted to the component (10) so that the outwardlydiffracted beams form substantially the same virtual image as theinwardly inputted beams but which is perceivable over a much largerarea.

In the example of FIG. 9B, A≈45° i.e. so that the fold SRG and exit SRGs(54, 56) are oriented at substantially 45 and 90 degrees to theincoupling SRG (52) respectively, with the grating period of the foldregion d₂=d/√{square root over (2)}. However, this is only an exampleand, in fact, the overall efficiency of the display system is typicallyincreased when A≥50°.

The above considers flat optical components, but a suitably curvedoptical component (that is, having a radius of curvature extendingsubstantially along the z direction) can be configured to function as aneffective lens such that the output beams 38(X) are and are no longer ashighly collimated and are not parallel, but have specific relativedirection and angular separations such that each traces back to a commonpoint of convergence—this is illustrated in FIG. 7G, in which the commonpoint of convergence is labelled Q. Moreover, when every image point isconsidered, the various points of convergence for all the differentactive image points lie in substantially the same plane, labelled 50,located a distance L from the eye (37) so that the eye (37) can focusaccordingly to perceive the whole image as if it were the distance Laway. This can be equivalently stated as the various output beamsforming substantially the same virtual version of the current displayimage as the corresponding input beams, but at the distance L from theeye (37) rather than at infinity. Curved optical components may beparticularly suitable for short-sighted eyes unable to properly focusdistant images.

Note, in general the “width” of the fold and exit zones does not have tobe their horizontal extent—in general, the width of a fold or exit zone(14, 16) is that zone's extent in the general direction in which lightis coupled into the fold zone 14 from the incoupling zone 12 (which ishorizontal in the above examples, but more generally is a directionsubstantially perpendicular to the grating lines of the incoupling zone12).

Returning to FIG. 2B, left and right input beams are guided though leftand right waveguides (10L, 10R) onto the left and right eyerespectively. Note that, for a transmissive arrangement in which thebeams are coupled into and exit the optical component on opposite sides,is does not matter if the waveguides (10L, 10R) move relative to theleft and right imaging components (15L/17L, 15R, 17R) as this does notchange the orientation of the output beams i.e. even if the opticalcomponents rotate or move, the angular relationship between the inputand output beams is unchanged (in this example, they remain parallel).It is only relative movement between the left components (15L/17L) andthe right components (15R/17R) that introduces binocular disparity. Thusall that is needed to maintain binocular parity of the left and rightimages is to ensure that angular alignment of the left and right imagingcomponents (15L/17L, 15R/17R) is preserved, which is achieved by housingthem at the same central location and further aided by the rigid supportstructure.

This is true whenever for any type of incoupling optics and outcouplingoptics (be they gratings or other structures) which are on oppositesides of the waveguide as this causes the waveguide to act like aperiscope where the angle of a light ray entering the waveguide is equalto the angle of the light ray exiting the waveguide. Further details ofthis effect are described in the Applicant's International PatentApplication PCT/US2014/016658, filed 17 Feb. 2014, which relates tocoupling light into waveguides in a near-eye display device in a mannerconfigured to be tolerant to misalignment of the waveguides with eachother and/or other optics. For example, one arrangement disclosedtherein provides a near-eye display device comprising one or morewaveguides, wherein each waveguide comprises a light input couplingconfigured to receive light at a first side of the waveguide to couplethe light into the waveguide, and a light output coupling configured toemit light from the waveguide at a second side of the waveguide, thesecond side of the waveguide being opposite the first side of thewaveguide.

The support structure in the central portion (4) is sufficiently rigidto ensure that, during normal use of the system (1), beams OBL outputfrom the left exit grating 16L of the left optical component 10L ontothe user's left eye remain aligned with beams OBR output from the rightexit grating 16R of the right optical component 10R onto the user'sright eye to within ½ milliradian of their intended alignment (i.e. thatfor which the correct stereoscopic image is perceived), at least asmeasured relative to the vertical direction. Note that alignment towithin 1 milliradian is acceptable in practice. As will be apparent inview of the foregoing, maintaining this level of angular alignmentensures alignment of the left and right images to within one pixel atleast in the vertical direction. Vertical disparity is generally beingmore perceptible to the HVS than horizontal disparity as discussed, buthorizontal alignment may nonetheless be preserved to the same precisionby some support structures. As will be apparent, a variety ofsufficiently stiff, lightweight materials can be used to make thesupport structure.

FIG. 10 shows another feature of the head mounted display. FIG. 10 is aview looking from the side of the head mounted display shown in FIG. 1.It shows one of the support extensions 6 and the mounting portion 4. Thewearer's ears are not shown in FIG. 10, but it will be understood that apart (90) of the support extension (6) fits over an ear of the user andextends horizontally therefrom towards the front of the user's face. Thedisplay (15) lies in a plane (92), which is shown to be vertical andsubstantially perpendicular to the support extension (6) in the figures.However, in general the display can be arranged in any orientation (e.g.the display panel can be even in horizontal position) depending on howthe folding optics of the light engine is implemented.

FIG. 10 also shows the optical component (10) and in particular showsthat the optical component (10) is not arranged vertically with respectto the supporting extension (6). Instead, the optical component (10)extends at an angle towards the user's eye. In FIG. 7, the vertical isshown by a dotted line and the angle is shown as an acute angle Θ.

The reason for this is shown in FIGS. 11 and 12. As shown in FIGS. 11and 12, the light engine 13 has an exit aperture EA. The exit aperturemay for instance be formed in a housing of the light engine, or apartition which separates the internal optics of the light engine fromthe waveguide. Light can only enter or exit the light engine 13 via theexit aperture EA. FIG. 11 shows how the light may behave when theoptical component is arranged truly vertically. Consider the incidentray labelled (I) which comes from a pixel (X) of the micro display (15)and is incident on the incoupling grating (12). For this incident ray(I), the angle of incidence is such that there is a reflected ray (R)which is reflected back through the imaging optics (17) and is incidenton the display (15). As the display (15) has some reflectivity at itssurface, a ghost reflection (r) is reflected off the micro display andformed by the imaging optics (17) onto the in-coupling grating (12) ofthe optical component. Thus, in addition to the desired ray (I) which isguided through total internal reflection through the optical componentand diffracted out to the user's eye (as output rays I′), there is aghost image formed by the reflective beam (R/r) which is also guidedthrough total internal reflection and ends up incident on the user's eye(as output rays r′). Although the light level of the ghost image mightbe small, nevertheless, it is an irritant to the user and destroys hisclarity of vision of the intended image.

FIG. 12 shows how this ghost image can be removed by angling the opticalcomponent (10) in the yz-plane at an angle Θ relative to the plane (92),with the bottom of the optical component (10) angled towards the user(i.e. so that the bottom of the optical component 10 is nearer the userthan the top of the optical component 10). In this case, the incidentray I is similarly reflected from the in-coupling grating (12), but thereflective beam R′ in this case is reflected at an angle which does nothit the lens of the optics (17). The angle Θ is sufficiently large thatthis is the case for all rays from X (which are collimated to form anincident beam IB) so that the version RB of the incident beam IB that isoutwardly reflected by the optical component (10) propagates entirelyclear of the optics (17). Thus, no ghost image of the pixel X is formed.

To ensure that no ghost images of any pixels are formed, this shouldhold true for all pixels on the display (recall, each pixel results in asingle respective beam), thus the angle Θ is dependent on thearrangement of the display 15, optics 17 and optical component 10relative to one another. When the optical component is tilted verticallytowards the user as in FIG. 12, it is sufficient for the angle Θ to belarge enough that the beams from the lower-most row of pixels arereflected clear of the collimating optics as these reflected beams willcome to the optics (17) than any other beams because they have thesmallest angles of incidence in the yz-plane.

Note that the above arrangement of the light engine 13 is just anexample. For example, an alternative light engine based on so-calledscanning can provide a single beam, the orientation of which is fastmodulated whilst simultaneously modulating its intensity and/or colour.As will be apparent, a virtual image can be simulated in this mannerthat is equivalent to a virtual image that would be created bycollimating light of a (real) image on a display with collimatingoptics.

The relevant factor with regards to preventing ghosting is the angle atwhich the collimated beams from the light engine meet the light guideplate, which is true whatever the configuration of the light engine.Ghosting will be eliminated provided beam back-reflected versions of thebeam cannot re-enter the light engine. Thus, ghosting is eliminatedwhenever the angle between the light engine and the optical component issuch that there will be no reflections from the plate back to the lightengine exit aperture at any angular values of the field of view of thelight engine.

Whilst in the above the optical components are tilted vertically towardsthe user, ghosting can be eliminated by angling each optical component,relative to the plane 92 in which the display 15 of the light enginelines, in the any direction, provided each optical component is tiltedrelative to the light engine by an angle large enough that all reflectedbeams clear the exit aperture.

The optical component (10) can be mounted at the angle Θ using anysuitable mounting mechanism; in particular it could be fixed intoportion of the frame which already tilted at this angle to providesupport for the optical component at this angle.

Note that the elimination of ghosting by tilting can be used in othertypes of display system, for example one in which beams from the samedisplay are coupled into left and right optical waveguide components sothat an image is perceived by both eyes from a single display, or inwhich a single waveguide is used to provide an image from a singledisplay to one eye only.

Whilst the above covers Surface Relief Gratings, the subject matter isapplicable to other structures for example other diffractive basedwaveguide displays, and reflective (non-diffractive) waveguide displays.

According to a first aspect, a wearable image display system comprises aheadpiece, a first and a second light engine, and a first and a secondoptical component. The first and second light engines are configured togenerate a first and a second set of beams respectively. Each beam issubstantially collimated so that the first and second set form a firstand a second virtual image respectively. The light engines are mountedon the headpiece. Each optical component is located to project an imageonto a first and a second eye of a wearer respectively and comprises anincoupling structure and an exit structure. The first and second sets ofbeams are directed to the incoupling structures of the first and secondoptical components respectively. The exit structures of the first andsecond optical components are arranged to guide the first and secondsets of beams onto the first and second eyes respectively. The opticalcomponents are located between the light engines and the eyes. Both ofthe light engines are mounted to a central portion of the headpiece.

In embodiments, the system may comprise a support structure mounted tothe central portion which supports the first and second light engines,the support structure more rigid than the headpiece.

The support structure may be sufficiently rigid to maintain verticalalignment between the first and second sets of beams to withinsubstantially one milliradian. In addition, horizontal alignment betweenthe first and second sets of beams may also maintained by the supportstructure to within substantially one milliradian. The support structurema for example formed of carbon fibre or titanium.

Each optical component may comprise a fold structure which manipulatesthe spatial distributions of the beams within the waveguide.

The optical components may be substantially transparent whereby a usercan see through them to view a real-world scene simultaneously with theprojected images.

The first and second sets of beams may be directed from first and secondexit apertures of the first and second light engine respectively, andthe optical components may be angled relative to the light engines suchthat any outwardly reflected versions of the beams propagate clear ofthe exit apertures.

The first and second images may differ from one another so that astereoscopic image is perceived by the wearer.

The first light engine may comprise a first display on which a firstimage is generated, and collimating optics arranged to generate thefirst set of beams from the first image on the first display; the secondlight engine may comprise a second display on which a second image isgenerated, and collimating optics arranged to generate the second set ofbeams from the second image on the second display.

The structures may be gratings, whereby the beams are diffracted ontothe eye.

The headpiece may comprise a frame, helmet or headband.

The optical components may for example be formed of glass or polymer.

According to a second aspect, a wearable image display system comprisesa headpiece, collimating optics, a first and a second display on which afirst and a second image is generated respectively, a first and a seconddisplay on which a first and a second image is generated respectively,and a first and a second optical component. The displays are mounted onthe headpiece. Each optical component is located to project an imageonto a first and a second eye of a wearer respectively and comprises anincoupling structure and an exit structure. The collimating optics isarranged to substantially collimate each image into respective beams andto direct the beams of the first and second images to the incouplingstructures of the first and second optical components respectively. Theexit structures of the first and second optical components are arrangedto diffract versions of the first and second images onto the first andsecond eyes respectively. The optical components are located between thecollimating optics and the eyes. Both of the displays and thecollimating optics are mounted to a central portion of the headpiece.

In embodiments, the optical components may be substantially transparentwhereby a user can see through them to view a real-world scenesimultaneously with the projected images.

The first and second images may differ from one another so that astereoscopic image is perceived by the wearer.

According to a third aspect, a wearable image display system comprises aframe, collimating optics, a first and a second display on which a firstand a second image is generated respectively, and a first and a secondoptical component. The displays mounted on the frame. Each opticalcomponent is located to project an image onto a first and a second eyeof a wearer respectively and comprises an incoupling grating and an exitgrating. The collimating optics is arranged to substantially collimateeach image into respective beams and to direct the beams of the firstand second images to the incoupling gratings of the first and secondoptical components respectively. The exit gratings of the first andsecond optical components are arranged to diffract versions of the firstand second images onto the first and second eyes respectively. Theoptical components are located between the collimating optics and theeyes. A support structure is mounted to a central portion of the frameand supports the first and second displays and the collimating optics,the support structure more rigid than the frame.

The support structure may be sufficiently rigid to maintain verticalalignment between the diffracted versions of the first and second imagesto within substantially one milliradian. Horizontal alignment betweenthe diffracted versions of the first and second images may also bemaintained by the support structure to within substantially onemilliradian.

Each optical component may comprise a fold grating which manipulates thespatial distributions of the beams within the waveguide.

The optical components may be substantially transparent whereby a usercan see through them to view a real-world scene simultaneously with theprojected images.

The first and second images may differ from one another so that astereoscopic image is perceived by the wearer.

According to a fourth aspect, a wearable image display system comprisesa headpiece, a light engine, and an optical component. The light engineis mounted on the headpiece and configured to generate beams, each ofthe beams being substantially collimated so that the beams form avirtual image. The optical component is located to project an image ontoan eye of a wearer and comprises an incoupling structure and an exitstructure. The beams are directed from an exit aperture of the lightengine to the in-coupling structure of the optical component. The exitstructure is arranged to guide the beams onto the eye. The opticalcomponent is located between light engine and the eye. The opticalcomponent is angled relative to the light engine such that any outwardlyreflected versions of the beams propagate clear of the exit aperture.

In embodiments, the light engine may comprise a display on which animage is generated, and collimating optics arranged to generate thebeams from the image on the display.

The structures may be gratings, whereby the beams are diffracted ontothe eye.

The optical component may be angled towards the wearer.

The optical component may comprise a fold structure which manipulatesthe spatial distributions of the beams within the waveguide.

The optical component may be substantially transparent whereby a usercan see through it to view a real-world scene simultaneously with theprojected image.

The optical component may comprise two such light engines, eachconfigured to generate a respective such virtual image, and two suchoptical components wherein the virtual images differ from one another sothat a stereoscopic image is perceived by the wearer.

The optical components may for example be formed of glass or polymer.

The light engine may be mounted to a central portion of the frame.

The headpiece may comprise a frame, helmet or headband.

According to a fifth aspect, a wearable image display system comprises aheadpiece, a display on which an image is generated, an opticalcomponent, and collimating optics. The display is mounted on theheadpiece and lies in a plane. The optical component is located toproject an image onto an eye of a wearer and comprises an incouplingstructure and an exit structure. The collimating optics is arranged tosubstantially collimate the image into beams and to direct the beams tothe in-coupling structure of the optical component. The exit structureis arranged to guide the beams onto the eye. The optical component isangled relative to said plane by an amount such that that any outwardlyreflected versions of the beams propagate clear of the collimatingoptics.

The structures may be gratings, whereby the beams are diffracted ontothe eye.

The optical component may be angled towards the wearer.

The optical component may comprise a fold structure which manipulatesthe spatial distributions of the beams within the waveguide.

The optical component may be substantially transparent whereby a usercan see through it to view a real-world scene simultaneously with theprojected image.

The optical component may for example be formed of glass or polymer.

According to a sixth aspect, a wearable image display system comprises aheadpiece; a first and a second display on which a first and a secondimage is generated respectively, a first and a second optical component,and collimating optics. The displays are mounted on the headpiece andlie in a plane. Each optical component is located to project an imageonto a first and a second eye of a wearer respectively and comprises anincoupling grating and an exit grating. The collimating optics isarranged to substantially collimate each image into respective beams andto direct the beams of the first and second images to the incouplinggratings of the first and second optical components respectively. Theexit gratings of the first and second optical components is arranged todiffract versions of the first and second images onto the first andsecond eyes respectively. The optical components is located between thecollimating optics and the eyes. Each optical component is angledrelative to said plane by an amount such that any outwardly reflectedversions of the beams propagate clear of the collimating optics.

The first and second images may differ from one another so that astereoscopic image is perceived by the wearer.

Although the subject matter has been described in language specific tostructural features and/or methodological acts, it is to be understoodthat the subject matter defined in the appended claims is notnecessarily limited to the specific features or acts described above.Rather, the specific features and acts described above are disclosed asexample forms of implementing the claims.

What is claimed is:
 1. An image display system comprising: a frame; afirst light engine configured to generate a first image; a second lightengine configured to generate a second image; a first optical componentconfigured to guide the first image to a first eye of a user of theimage display system; and a second optical component configured to guidethe second image to a second eye of the user of the image displaysystem, wherein the first light engine and the second light engine aremounted to a central portion of the frame.
 2. The image display systemof claim 1, wherein the first light engine and the second light engineare configured to generate the first image and the second imagedifferently so that the user perceives a stereoscopic image.
 3. Theimage display system of claim 1, wherein the first light engine and thesecond light engine comprise respective micro-displays.
 4. The imagedisplay system of claim 3, wherein the respective micro-displayscomprise liquid crystal on silicon displays, transmissive liquid crystaldisplays, or light-emitting diode displays.
 5. The image display systemof claim 3, wherein the respective micro-displays define a verticalplane, and the first optical component and the second optical componentare tilted relative to the vertical plane.
 6. The image display systemof claim 5, wherein the first optical component and the second opticalcomponent are tilted toward the first eye of the user and the second eyeof the user, respectively.
 7. The image display system of claim 1, thefirst optical component and the second optical component comprisingdiffractive structures configured to guide the first image and thesecond image, respectively.
 8. The image display system of claim 1,embodied as a head-mounted display having a headpiece comprising theframe.
 9. The image display system of claim 8, the headpiece furthercomprising supporting extensions configured to fit over respective earsof the user.
 10. An image display system comprising: a frame; aplurality of displays configured to generate a plurality of images; andat least one optical component configured to guide the plurality ofimages to at least one eye of a user of the image display system,wherein the plurality of displays are mounted to a central portion ofthe frame.
 11. The image display system of claim 10, the central portionof the frame being configured to fit over a bridge of the user's nose.12. The image display system of claim 10, the central portion forming ahousing that houses the plurality of displays.
 13. The image displaysystem of claim 10, the central portion of the frame comprising asupport structure that is relatively more rigid than a remainder of theframe.
 14. A system comprising: a frame having a rigid central supportstructure; a first display configured to generate a first image; asecond display configured to generate a second image; a first opticalcomponent configured to guide the first image to a first eye of a userof the system; and a second optical component configured to guide thesecond image to a second eye of the user of the system, wherein thefirst display and the second display are mounted to the rigid centralsupport structure.
 15. The system of claim 14, wherein the first displaydefines a vertical plane, and the first optical component is tiltedrelative to the vertical plane such that a lower portion of the firstoptical component is relatively closer to the first eye of the user thanan upper portion of the first optical component.
 16. The system of claim15, wherein the first optical component is located between the firstdisplay and the first eye of the user when the system is in use.
 17. Thesystem of claim 14, wherein the first optical component is configured toproduce a diffracted version of the first image and the second opticalcomponent is configured to produce a diffracted version of the secondimage.
 18. The system of claim 17, wherein the rigid central supportstructure is relatively more rigid than at least one other portion ofthe frame.
 19. The system of claim 18, wherein the rigid central supportstructure is sufficiently rigid to maintain vertical alignment betweenthe diffracted version of the first image and the diffracted version ofthe second image to within substantially one milliradian.
 20. The systemof claim 18, wherein the rigid central support structure is sufficientlyrigid to maintain horizontal alignment between the diffracted version ofthe first image and the diffracted version of the second image to withinsubstantially one milliradian.